The cleaning of integrated circuit (IC) substrates, such as silicon wafers, with metal-free alkaline solutions to remove organic and metal contamination is widely practiced. One commonly used alkaline solution of this type is known as SC-1 or RCA-1 and comprises a hot aqueous mixture of ammonium hydroxide, hydrogen peroxide, and water (1:1:5 of 30% H.sub.2 O.sub.2, 28% NH.sub.4 OH and H.sub.2 O) to remove organic impurities and copper contamination from a wafer surface. Various cleaning tasks can be accomplished with SC-1, among these, the cleaning of silicon wafers immediately after their fabrication, the cleaning of such wafers immediately prior to gate oxide growth, the removal of oxide etch residues later in the IC processing sequence, and selective etching and resist particulate removal.
Treatment of the wafer surfaces with the hot SC-1 or RCA-1 solution is generally followed by a hot acid solution known as SC-2 or RCA-2 to remove metals untouched by the SC-1 or RCA-1 solution. This hot acid solution SC-2 comprises hydrogen peroxide, hydrochloric acid and water (1:1:5 of 30% H.sub.2 O.sub.2, 37% HCl and H.sub.2 O).
Both solutions, SC-1 and SC-2 contain hydrogen peroxide. The purpose of the hydrogen peroxide is to protect the silicon metal from exposure to strong acids or bases by continuously forming a protective oxide layer in order to prevent etching or roughening of the silicon surface.
It is, however, necessary for the wafer surfaces to be oxide-free to be suitable for further processing where an oxide surface is not wanted. Usually, it is then necessary to remove the protective oxide layer formed by the hydrogen peroxide in the cleaning solutions. As an example of a material commonly used to remove such protective oxide layer, there may be mentioned HF.
The presence of hydrogen peroxide in the formulations imparts an inherent instability to these solutions. Such solutions typically exhibit peroxide half-lives of less than one hour at 70.degree. C. The hydrogen peroxide in the SC-1 solution in the presence of certain metals, particularly copper and iron, becomes unstable and decomposes in rapid exothermic fashion leading to potentially dangerous conditions. The hydrogen peroxide has a low tolerance for metal contamination. Additionally, the decomposed hydrogen peroxide drops the concentration of the hydrogen peroxide leading to the possibility of silicon etching producing wafers that are not acceptable for IC manufacture. Thus, the decomposed hydrogen peroxide needs to be replenished and this changes the solution composition thereby varying the cleaning properties of the solution. In addition, the inherently high pH of the hydrogen peroxide solution presents undesirable safety and environmental concerns.
Since the introduction of the SC-1 or RCA-1 solution, there have been proposals for using basic materials other than ammonium hydroxide to clean wafer surfaces. For example, quaternary ammonium hydroxide compounds, such as tetramethylammonium hydroxide (TMAH) or trimethyl-2-hydroxyethyl ammonium hydroxide (choline) have been reported in Japanese Patent Publications No. 3-93229 and 63-114132; U.S. Pat. Nos. 4,239,661; 4,964,919 and 5,259,888 and European Patent Publication No. 496605, for example. It is to be noted that the wafer roughness values mentioned in U.S. Pat. No. 4,964,919 are unacceptable for high density integrated circuit manufacture. Moreover, U.S. Pat. No. 5,207,866 describes a case where a quaternary amine without hydrogen peroxide present is used to anisotropically etch the silicon 100 face of wafers.
Without hydrogen peroxide present, none of the above mentioned alkaline or quaternary ammonium hydroxide-based cleaners can produce the acceptable roughness levels achieved by the present invention and necessary for high density integrated circuit manufacture.
It is an object of this invention to provide a cleaning solution for cleaning wafer substrates of metal contamination without increasing surface microroughness which cleaner composition does not require the use of hydrogen peroxide to provide a protective oxide layer. A further object of this invention is to provide a cleaner composition for cleaning wafer substrates of metal contamination without increasing surface microroughness and leaving an oxide-free wafer surface making the surface suitable for further processing where an oxide surface is not wanted. A still further object of this invention is to clean such wafer surfaces of metal contamination without requiring an acid treatment step or the use of materials, such as HF, used to remove oxide surfaces. An additional aspect of this invention is to provide a process for cleaning such wafer surfaces of metal contamination without increasing wafer surface microroughness by using only a single cleaning solution. Yet another object of this invention is to provide a process and composition for cleaning such wafer surfaces of metal contamination without increasing wafer surface microroughness using an aqueous alkaline solution, and more particularly, using an aqueous quaternary ammonium hydroxide solution free of hydrogen peroxide or other oxidizing agents. An even still further object of this invention is to provide such a process and composition for cleaning vias in processed wafers of resist and etching residue, particularly metal contamination. Yet another object of this invention is to provide such a process and alkaline cleaning composition for cleaning wafers and producing a roughness of less than about 25 Angstrom as the average distance in the Z direction between wafer peak heights and valleys.